1. Field of the Invention
The present invention relates to a catalyst for a fuel cell, a method of preparing the same, a membrane-electrode assembly for a fuel cell, and a fuel cell system including the same. More particularly, the present invention relates to a cathode catalyst having an improved catalytic activity and selectivity for reduction of an oxidant, and a membrane-electrode assembly including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
A fuel cell is a clean energy source that can replace fossil fuel energy sources. It includes a stack composed of unit cells and produces various ranges of power output. Since it has a four to ten times higher energy density than a small lithium battery it has been highlighted as a small portable power source.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte membrane fuel cell has an advantage of a high energy density and energy conversion efficiency, but it also has problems in the need to carefully handle hydrogen gas and the requirement of additional devices such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the polymer electrolyte membrane fuel cell, but it has the advantages of easy handling of a fuel, being capable of operating at room temperature, and no need for additional fuel reforming processors. Therefore, it has been acknowledged as an appropriate system for a portable power source for small electrical devices.
In the above fuel cells, the stack that generates electricity substantially includes several to scores of unit cells stacked in multiple layers, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
A fuel is supplied to the anode and is adsorbed on catalysts of the anode, and the fuel is oxidized to produce protons and electrons. The electrons are transferred into the cathode via an external circuit, and the protons are also transferred into the cathode through the polymer electrolyte membrane. An oxidant is supplied to the cathode, and the oxidant, protons, and electrons are reacted on catalysts of the cathode to produce electricity along with water.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.